Throughout the pneumatic art, nozzles and co-operating flappers are utilized to generate an output pressure that is proportional to an applied input. Because these nozzle-flapper units form a delicate balanceable assembly whose operating region involves relative motions between flapper and nozzle of less than 0.001 inches, and because of the varying resonant frequencies of the other instrument components that co-operate with the nozzle and flapper, problems with vibrations can arise under certain applications producing resultant instrument output errors.
In the past, considerable effort has been directed towards minimizing these vibration-induced errors by means of damping or otherwise. For example, one method is to build an instrument in which the resonant frequencies of the functionally interrelated components are well above the excitation vibrational frequencies. In this manner, the effects of vibration are effectively reduced. However, owing to design limitations in being able to select components of low mass and high spring rate while still providing useful output under various applications, such an approach is not practical.
Other prior art attempts have involved the use of tuned mechanical structures, instrument isolation mounts, counterbalance and viscous damping means to diminish the effects of vibration. Such additional structure increases the overall complexity of the instrument and hence its manufacturing expense.
U.S. Pat. No. 3,275,238 discloses using a filter for the input air supply to the nozzle by selecting a length of tubing that is one-quarter wave length the resonant frequency of the condition responsive member. In such a manner, resonant vibrations travel down the tubing and are reflected back to the nozzle 180.degree. out of phase with the resonant vibration, thereby resulting in reciprocal cancellation of both vibration waves. In U.S. Pat. No. 3,426,970, there is proposed the creation of a "cushion of air" between the flapper and the nozzle by designing a nozzle structure with two surfaces having a fixed dimensional relationship to the position of the flapper.
While both of the aforementioned patents are primarily concerned with vibrational problems, they propose solutions which are only effective in reducing self-induced vibrations resulting from the fluid dynamics of the nozzle air blast impinging on the flapper, and which are almost totally ineffective in diminishing the effects of external vibrations imposed on the instrument. Thus, it is apparent that the need exists for a simple, inexpensive structure used in conjunction with a nozzle-flapper system that is capable of withstanding the adverse effects of externally induced vibrations.